1. Field of the Invention
The present invention relates to methods for forming thin films.
2. Description of the Related Art
Metal fluorides such as aluminum fluoride (AlF3) and magnesium fluoride (MgF2) have been used for optical elements (lenses and mirrors) for visible light and for antireflection coatings formed on the optical elements.
Fluoride antireflection coatings (thin films) are formed mainly by vacuum evaporation. Vacuum evaporation is a method that can be performed with simple equipment and can form a thin film on a substrate having a large area at a high speed, and is thus superior in productivity. However, the thickness of a film formed by vacuum evaporation is liable to be reduced in portions distant from the deposition source, relative to the thickness near the deposition source. Accordingly, a substrate rotation mechanism is used for highly precisely controlling the thickness so that an uneven film is not formed depending on the relative position with respect to the deposition source. In addition, it has been difficult to develop an automatic machine for vacuum evaporation.
In order to enhance the adhesion between the film and the substrate and to reduce the absorption of the film, furthermore, a heating mechanism is undesirably required for heating the substrate to about 300° C.
Accordingly, sputtering is attracting attention as a method for forming fluoride antireflection coatings (thin films). Sputtering is superior to vacuum evaporation in reproducibility, and in reducing the unevenness of the film thickness and forming films at a low temperature. For forming a film by sputtering, charged particles are caused to collide with a target of a deposition material by applying a voltage, and particles of the deposition material produced by the collisions fly and are deposited on a substrate. When a fluoride antireflection coating is formed by sputtering, however, the reactivity between the sputtering material and fluorine may be insufficient, or the substrate may be easily damaged by charged particles. As a result, the fluoride antireflection coating on the substrate absorbs light having wavelengths larger than the band gap, and thus the antireflection property of the metal fluoride film is undesirably reduced.
Some solutions for this disadvantage have been proposed. For example, Japanese Patent Laid-Open No. 4-289165, which takes reactivity into account, discloses a method in which a metal fluoride is controlled to have a stoichiometric composition. In this method, a mixture of a fluorine-based gas, such as F2 or CF4, and an inert gas, such as Ar, is used as a sputtering gas so as to supply fluorine (F), which is often lacking, to the target. Japanese Patent Laid-Open No. 2002-47565, which takes into account the damage to the deposited film from plasma, discloses a sputtering apparatus using a cylindrical target to which a direct current is applied. In this apparatus, a sputtering gas is introduced through the bottom of the cylinder, and a reaction gas is introduced from a side near to the substrate.
However, when a metal fluoride is deposited on a substrate using F2 gas as a reaction gas, cost is increased. F2 gas is expensive. In addition, F2 gas is harmful to the human body, and if it is used, facilities for safety measures are required. Thus, the use of F2 gas is disadvantageous in terms of production cost.
Also, if a mixture of a fluorocarbon gas, such as CF4, and an inert gas, such as Ar, is used for sputtering, elemental carbon or a carbon compound may be drawn into the deposited film. This increases optical loss and thus negatively affects the quality of the optical element in terms of optical properties.
Thus, known methods for thin film deposition have not reached a satisfactory level. The present invention provides a safe, inexpensive method for forming a metal fluoride film exhibiting low absorption in the visible region, by sputtering.